The reduction of nitrogen oxide (NO.sub.x) emissions is a key environmental objective for the new millennium. Atmospheric emissions of NO.sub.x not only have detrimental effects on human health, but also have adverse effects on the ecosystems in which we live. Exposure to high levels of NO.sub.x produces immediate acute effects and prolonged exposure above ambient levels leads to bronchitis, pneumonia, susceptibility to viral infections and alterations to the immune system. In addition to these direct health effects, NO.sub.x contributes to urban smog by reacting with volatile organic compounds in the atmosphere to form ozone, and also causes acid precipitation.
The United States anthropogenic NO.sub.x emissions are estimated at 23 million tons for 1992. Despite improved control strategies, this represents a 5% increase since 1983. The sources of NO.sub.x pollutants can be classified into two categories: stationary and non-stationary. Non-stationary sources (motorized vehicles) contributed 45% of the total United States NO.sub.x emissions for 1992. The remaining 55% of the pollutants can be attributed to stationary sources such as power plants (29%), internal combustion engines (11%), industrial boilers (8%), process heaters (3%), gas turbines (1%) and other sources (3%). Since nearly all of these NO.sub.x emissions arise from the combustion of fossil fuels, the development of improved methods by which NO.sub.x can be removed from exhaust gases is of critical importance.
Several catalytic approaches for the removal of NO.sub.x from exhaust gases have been developed in the past few decades. Direct decomposition of NO.sub.x involves the decomposition of NO.sub.x into molecular nitrogen and oxygen, but the current generation of catalysts for this reaction are not active nor robust enough to be applied in practice e.g. the zeolite Cu-ZSM-5. Selective catalytic reduction of NO.sub.x using ammonia as a reducing agent has been industrially applied for several decades, but the use of ammonia leads to high equipment costs due to its corrosive nature, and also gives rise to the phenomenon known as "ammonia slip", where unreacted ammonia is exhausted to the atmosphere. The use of hydrocarbons as reducing agents is favored over ammonia, and many catalytic systems for NO.sub.x reduction with hydrocarbons have been developed. These systems typically employ C.sub.2 or higher hydrocarbons.
The large worldwide reserves of natural gas and the availability of methane at gas fired power plants makes methane an attractive reductant for stationary applications. However, over most catalysts methane preferentially reacts with oxygen present in the feed stream before reducing NO.sub.x. Some of the few catalysts that do promote NO.sub.x reduction with methane in an oxidizing atmosphere are zeolites such as ZSM-5 or ferrierite zeolites exchanged with Co, Mn, Ni, In, Ga or Pd ions. Near complete conversion of NO.sub.x to N.sub.2 is achieved when an excess of methane is used at low space velocities.
Currently, catalysts that do not require an ammonia reductant for NO.sub.x reduction, such as Co-ZSM-5, do not have sufficient catalytic activity for industrial purposes, especially in the presence of water. There exists a need for the development of new materials that do not themselves promote the emission of harmful compounds into the atmosphere.